DISORDERS OF POTASSIUM BALANCE DR. ALI ABDUL-RAHMAN YOUNIS

DISORDERS OF POTASSIUM BALANCE Potassium is the major intracellular cation. Changes in the distribution of potassium between the ICF and ECF compartments can alter plasma potassium concentration, without any overall change in total body potassium content. Potassium is driven into the cells by extracellular alkalosis and by a number of hormones, including insulin, catecholamines (through the β2recep-tor) and aldosterone. Any of these factors can produce hypokalaemia. Whereas extracellular acidosis, lack of insulin, and insufficiency or blockade of catecholamines or aldosterone can cause hyperkalaemia due to efflux of potassium from the intracellular compartment.

Functional anatomy and physiology of renal potassium handling In the steady state, the kidneys excrete some 90% of the daily intake of potassium, typically 80–100 mmol/day. Potassium is freely filtered at the glomerulus; around 65% is reabsorbed in the proximal tubule and a further 25% in the thick ascending limb of the loop of Henle. Little potassium is transported in the early distal tubule but a significant secretory flux of potassium into the urine occurs in the late distal tubule and cortical collecting duct to ensure that the amount removed from the blood is proportional to the ingested load.

Functional anatomy and physiology of renal potassium handling The most important factor in the acute and chronic adjustment of potassium secretion is aldosterone. A negative feedback relationship exists between the plasma potassium concentration and aldosterone. In addition to its regulation by the renin–angiotensin system , aldosterone is released from the adrenal cortex in direct response to an elevated plasma potassium. Aldosterone then acts on the kidney to enhance potassium secretion, hydrogen secretion and sodium reabsorption, in the late distal tubule and cortical collecting ducts. The resulting increased excretion of potassium maintains plasma potassium within a narrow range (3.3–4.7 mmol/L). Factors that reduce angiotensin II levels may indirectly affect potassium balance by blunting the rise in aldosterone that would otherwise be provoked by hyperkalaemia. This accounts for the increased risk of hyperkalaemia during therapy with ACE inhibitors and related drugs.

Feedback control of plasma potassium concentration.

Presenting problems in disorders of potassium balance

Hypokalaemia Aetiology and clinical assessment Patients with mild hypokalaemia (plasma K 3.0–3.3 mmol/L) are generally asymptomatic, but more profound reductions in plasma potassium often lead to muscular weakness and associated tiredness. Ventricular ectopic beats or more serious arrhythmias may occur and the arrhythmogenic effects of digoxin may be potentiated. Typical ECG changes occur, affecting the T wave in particular . Functional bowel obstruction may occur due to paralytic ileus. Long-standing hypokalaemia causes renal tubular damage (hypokalaemic nephropathy) and interferes with the tubular response to ADH (acquired nephrogenic diabetes insipidus), resulting in polyuria and polydipsia.

Investigations Measurement of plasma electrolytes, bicarbonate, urine potassium and sometimes of plasma calcium and magnesium is usually sufficient to establish the diagnosis. If the diagnosis remains unclear, plasma renin should be measured. Levels are low in patients with primary hyperaldosteronism and other forms of mineralo-corticoid excess, but raised in other causes of hypokalaemia. The cause of hypokalaemia may remain unclear despite the above investigations when urinary potassium measurements are inconclusive and the history is incomplete or unreliable.

Investigations Many such cases are associated with metabolic alkalosis, and in this setting the measurement of urine chloride concentration can be helpful. A low urine chloride (<30 mmol/L) is characteristic of vomiting (spontaneous or self-induced, in which chloride is lost in HCl in the vomit), while a urine chloride >40 mmol/L suggests diuretic therapy (acute phase) or a tubular disorder such as Bartter’s or Gitelman’s syndrome. Differentiation between occult diuretic use and primary tubular disorders can be achieved by performing a screen of urine for diuretic drugs.

Management Treatment of hypokalaemia involves first determining the cause and then correcting this where possible. If the problem is mainly one of redistribution of potassium into cells, reversal of this (for example, correction of alkalosis) may be sufficient to restore plasma potassium without providing supplements. In most cases, however, some form of potassium replacement will be required. This can generally be achieved with slow-release potassium chloride tablets, but in more acute circumstances intravenous potassium chloride may be necessary.

Management The rate of administration depends on the severity of hypokalaemia and the presence of cardiac or neuromuscular complications, but should generally not exceed 10 mmol of potassium per hour. In patients with severe, life-threatening hypokalaemia, the concentration of potassium in the infused fluid may be increased to 40 mmol/L if a peripheral vein is used, but higher concentrations must be infused into a large ‘central’ vein with continuous cardiac monitoring.

Management In the less common situation where hypokalaemia occurs in the presence of systemic acidosis, alkaline salts of potassium, such as potassium bicarbonate, can be given by mouth. If magnesium depletion is also present, replacement of magnesium may also be required for hypokalaemia to be corrected since low cell magnesium can enhance the mechanism for tubular potassium secretion, causing ongoing urinary losses. In some circumstances, potassium-sparing diuretics, such as amiloride, can assist in the correction of hypokalaemia, hypomagnesaemia and metabolic alkalosis, especially when loop or thiazide diuretics are the underlying cause.

Hyperkalaemia Aetiology and clinical assessment Hyperkalaemia can present with progressive muscular weakness, but sometimes there are no symptoms until cardiac arrest occurs. ECG changes: Peaking of the T wave is an early ECG sign, but widening of the QRS complex presages a dangerous cardiac arrhythmia.

Investigations Measurement of electrolytes, creatinine and bicarbonate, when combined with clinical assessment, usually provides the explanation for hyperkalaemia. In aldosterone deficiency, plasma sodium concentration is characteristically low, although this can occur in many causes of hyperkalaemia. Addison’s disease should be excluded unless there is an obvious alternative diagnosis.

Management Treatment of hyperkalaemia depends on its severity and the rate of development. In the absence of neuromuscular symptoms or ECG changes, reduction of potassium intake and correction of underlying abnormalities may be sufficient. However, in acute and/or severe hyperkalaemia (plasma K >6.5–7.0 mmol/L) more urgent measures must be taken. If ECG changes are present, the first step should be infusion of 10 mL 10% calcium gluconate to stabilise conductive tissue membranes .

Management Measures to shift potassium from the ECF to the ICF should also be taken, as they generally act rapidly and may avert arrhythmias. Ultimately, a means of removing potassium from the body is generally necessary. When renal function is reasonably preserved, loop diuretics (accompanied by intravenous saline if hypovolaemia is present) may be effective; In established renal failure, ion-exchange resins acting through the gastro-intestinal tract and urgent dialysis may be required.